The present invention relates to a system for controlling the fuel injection of an automotive engine in dependence on a throttle opening degree and engine speed. In a known fuel injection system, a basic fuel injection pulse width Tp is calculated in dependence on throttle opening degree O and engine speed N. The basic pulse width Tp are stored in a table shown in FIG. 5 and are derived for controlling the fuel injection during the operation of the engine. The basic fuel injection pulse width Tp is corrected in dependence on various factors such as engine speed, pressure in an intake passage, coolant temperature and vehicle speed, so that air-fuel mixture is prevented from becoming rich or lean (see for example, Japanese Patent Laid Open No. 55-32913).
There is an engine having an idle speed control valve provided in a bypass around a throttle valve. The idle speed control valve may be used for adjusting the amount of intake air at a low engine speed or at idling of the engine. In the air-fuel ratio control system where the air-fuel ratio is determined by throttle valve opening degree and engine speed, mass air flow or intake air pressure is not directly measured. Accordingly, if an opening area or cross sectional area 1 of the bypass is changed by the idle speed control valve to change the amount of intake air, the air-fuel ratio is deviated from a desired value.